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2. archery

Dealer Workbench
Tuning Twin Cam Systems
By Larry Wise
M
y first compound bow was a Jennings Trail Boss
two wheeler that I bought secondhand in 1976. I
really didn’t do much to it because I didn’t know
anything about the compound other than it had some “letoff ” and was much faster than my recurve. So I just shot it.
That December, I ordered and received a new Jennings
Arrow Star four wheeler. It was smooth to draw and aim and
had all the bells and whistles of the latest design. I shot it
with fingers that first winter and competed in the league
shoots at my local club. I wasn’t very good. I picked up a
little target panic but there was no way I was going to give
up; after all, I’d been shooting since I was eight years old and
really enjoyed it.
The real test came in the spring, when I started shooting
outdoors. That time, finger shooting didn’t go well either,
so the local Jennings sales rep, Henry Fulmer, gave me a
Stanislawski back tension release aid and my shooting took
off to new heights. Things were going really well until one
June day, when both cables broke.
That was the day I started learning how to tune the
compound. In fact, I learned how to rebuild a compound by
helping to install the new cables and reset the wheel timing,
draw weight and draw length. I had to start from scratch and
get sighted in again and by the following winter, I shot my
first 300 (60 arrows) on a 3 inch white spot target.
Over the next two years, I learned a lot from the guys
I shot with and against: guys like Jack Cramer, Ron Walker
and Mike Leiter. In fact, the four of us have become lifelong
friends. I also learned much from a man you may recall:
Tom Jennings. Since then, I’ve learned more and more as
newer cam systems have been developed but much of what
I learned in those early days still applies and I present it all
here for dealers to use as a guide for setting up bows for their
customers. I’m sure you have some preferred methods of
The Elite binary system is comprised of two symmetric cams
that are timed and shot at the factory to ensure the bow meets
advertised specifications. Elite uses a bumper post that stops with
contact on the limb as opposed to a stop on the cam draw-length
module. The interchangeable module is visible on the bottom cam,
which is on the right in this photo.
your own but it’s always good to cross-reference your own
ideas with others in case there’s something new that you
can adopt for your own use. A few new ideas may be all it
takes to keep customers happy and coming back into your
shop.
General Tuning Guide for Compound Bows
One of my job at this year’s ATA Show was to visit bow
manufacturers’ booths to glean new, if any, tuning techniques; take notes and present the information in a usable
form for all dealers who read ArrowTrade.
THE SYMMETRIC TWO CAM SYSTEM
I’ll start with the symmetric two cam system used on
Bowtech, Prime and Elite bows. My good friend Mike Derus
of Elite Archery in Henrietta, New York (elitearchery.com)
has always been a good source of information, so I called
him about the most recent models of Elite cams. Derus was
happy to inform me about Elite’s new Victory target model
twin cam. The Victory is 39 inches axle-to-axle, with a 7
inch brace height and modular binary cams to yield a 26 to
32 inch draw length range. Elite has two other binary cam
systems that produce more stored energy than the Victory
model.
Elite bows are equipped with a two track cam design, as
are most binary type systems. In the beginning days of two
wheel compounds, one end of each buss cable was attached
directly to the axle while the other end was reeled onto the
smaller diameter cable track of the cams. The binary system, however, attaches both ends of the cable to the cable
track on the cam; one end reels onto the module track while
the other end reels out cable into the system. The result is a
closed system that links both cams together; the cams roll in
sync with each other. Derus stressed that the system is fast,
accurate and less affected by cable or string stretching than
other systems.
Elite Archery sets the cam timing at the factory. In other
words, it makes sure that the cams are rolling in a synchronized manner. As you can assume, the cam timing is set by
twisting one or both cables (a bow press is required). The
factory shoots each bow and checks the cam timing and the
speed it generates. If the bow doesn’t meet the advertised
specs, then it is rebuilt so it does.
The limbs, a really important part, are a matched set
machined from Gordon glass blanks. The pair for each bow
is selected to be within 1 pound of deflection force of each
other and then installed at the factory.
SETUP: The basic starting point for Elite bows begins
with the arrow rest height set so the arrow is at the mounting hole level. A 1/4 inch nocking point setting for the top of
the nock will enable the bottom of the arrow to be level with
the arrow rest while the center shot setting is recommended
as .75 to .80 of an inch from the riser. Check the factory
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The new Mathews No-Cam system fits into the symmetric cam
category, since the top and bottom cams are mirror images of one
another. The large diameter bowstring wheel is round with the
axle at the center, thus prompting the name “No-Cam.” The cam
action is achieved on the smaller cable side of the cam system,
where a draw length module is also fitted. The result of this design
is a smooth drawing bow.
specs for each bow you have to adjust or repair.
The Elite binary cams are designed to provide level nock
travel. This makes good arrow flight easier to achieve and
that, in turn, means bow tuning is much simpler.
Prime bows feature a symmetric three track cam system.
The outer two tracks on the Prime cam are identical string
tracks that split the string load equally on either side of the
middle cable track. A small yoke joins the two strings into
one a short distance off of the cam. Draw length is controlled
by a center-track module system.
THE ASYMMETRIC CAM SYSTEM
The hybrid or “asymmetric” cam system has been
around for about 25 years. The original design, introduced
by Rex Darlington of Darton Bows, has caught on across the
industry and developed into one of the most accurate shooting systems ever.
The hybrid system is designed with a two track cam
on the top limb and a three track cam on the bottom. The
cable that reels onto the bottom center track is attached to
the top axle via a split-yoke cable system. The other cable,
a “slave” or control cable, is attached to the top cable track
so that it reels onto the cam. It attaches to the bottom cam
on the far right track (for right-handers) on the front or
These two pictures show the top and bottom cams of the Darton
symmetric system. Designer Rex Darlington uses the yoke system
to attach the cables to the small-diameter take-up wheels and also
a modular system to change draw length across a 7 inch span.
The bottom cam on this PSE hybrid cam system utilizes three
tracks to control bowstring reel-out and cable take-up. The proper
amount of both yields a level nock-travel path. It also features a
white string-side timing mark to show cam rotation relative to the
cable. At right above, the bottom cam on the PSE Freak SP is asymmetric and employs a sliding crescent-shaped draw module and a
draw-stop post to fit draw lengths from 27 to 33 inches. There are
also more timing marks on this side of the cam that provide a more
accurate reference for setting cam synchronization.
string-side facing the shooter so that it reels out into the system like the bowstring does but at a lesser rate.
Today’s computer technology allows for the design of all
of these tracks, two on the top and three on the bottom, to
control the intake and outlay of the cables and string in such
a manner as to create level nock travel during the power
stroke. In other words, the bow shoots arrows level across
the arrow rest, making the bow easy to tune. The two cam
design gives high speed, too, as these systems are capable of
producing arrow speeds in excess of 350 fps.
I communicated with Allen Rasor, PSE engineer, about
the newest PSE cams. He pointed out that when the bows
are assembled at the factory, each has the wheel timing set
according to the prescribed timing marks on both cams.
These marks indicate where each cable should be located
relative to each cam when the bow is in the brace position.
This will ensure that the cams roll in a synchronized manner
to their full draw position.
There are also timing marks on the back side of the
cams that are a bit more accurate. These marks show a range
within which the cables should be located. Dealers should
check these markings when they first remove the bow from
the box. As with other cable systems, the timing can be reset
by twisting cables; a bow press is required. PSE has released
several tech bulletins, which provide tuning steps for both
the Drive Cam and Center Pull Hybrid systems, to dealers.
If you don’t have them, contact PSE to get them; they are
quite helpful.
One thing PSE does, according to Rasor, is design cams
that can be adjusted across a wide span of draw lengths. The
38 inch axle-to-axle Freak SP, for instance, has an adjustment range of 6 inches from 27.5 to 33 inches in half-inch
increments, all on the same cam. Adjusting this draw length
module can be done without the use of a bow press.
Weight ranges span about 10 pounds on any given PSE
bow. Check the specs for other brands that you may deal
with. The recommendation for PSE bows is not to lower
the weight by fewer than four turns on each adjustment
bolt. This is according to the bow setup DVD that PSE has
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produced for its bows; it’s worth getting and showing in
your shop.
Regardless of the line of bows you sell, contact the
manufacturer for any tech articles it might have if you can’t
find them online.
INITIAL BOW SETUP: The starting point for an arrow
rest on a PSE bow is indicated by lines machined on the
arrow shelf and the side of the riser. The rest must be
adjusted so the arrow sits parallel to both the center shot line
on the shelf and the horizontal line across the rest-mount
hole. That makes it simple for anyone to do. You can use
this as a guide for any other bow regardless of whether it has
lines or not.
The nocking point should locate the bottom of the arrow
level with the arrow rest and perpendicular to the bowstring.
That means the top of the nock will be about 3/16 to 1/4 of
an inch above level as indicated by a bow square. A D-loop
is recommended and should be tied onto the bowstring with
the widely used Lark’s Head knot directly above and below
the arrow nock.
As with all the other bows I viewed at the show, after
setup, an arrow should be shot through paper from a 5 to 8
yard distance. This will verify the nocking point and center
shot location as well as help identify the proper arrow spine
for the bow.
TUNING BEYOND SETUP
After the arrow rest, nocking point, peep and bow sight
are installed, the tuning steps to follow for the different cam
types are all about the same. After all, you’re launching
arrows from a bowstring to a target, so getting them to fly
consistently with a high degree of stability requires a specific set of techniques. The following tuning guide will help
you get your customers’ bows shooting well with, I hope, a
minimum of time spent.
It should be obvious that bow tuning hasn’t changed
much over the last 10 years. In fact, most of what I did to
tune my Jennings T-Star back in the 1980s, I’m still doing
today. The major difference is that I don’t have to keep doing
it to the same bow over and over as the season progresses;
today’s bows stay in tune for a much longer period of time.
The string and limb materials used, design features, assembly techniques and factory testing make bows much more
ready to shoot right out of the box and far more likely to stay
in tune.
However, they still need some tuning to get them really
One very helpful tuning feature on PSE bows is the pair of reference lines machined into the riser. The arrow should be parallel to
and set just under the upper line on the side of the riser when the
arrow rest is correctly positioned. At the same time, the center shot
position should be set so that the arrow runs parallel to the bottom
line on the arrow shelf. This is very helpful during the initial setup
of the bow.
ready for target shooting or bowhunting because no factory
can truly personalize a bow for a particular individual. Use
the following tuning steps and you’ll be able to put the finishing touches to any bow you sell.
NOCK FIT: Proper fit can be tested by pulling the arrow
with your index finger and thumb. A slight tug should “pop”
the nock off the string. If you hold the bow so the string is
horizontal and the arrow falls off by its own weight, then the
nock fit is too loose.
In either case, one of two remedies must be employed. If
your nocks fit too tightly, you must either reduce the diameter of the bowstring or increase the throat size of your nocks.
Manufacturers are paying close attention to this these days
and I seldom encounter a problem with nock fit but check it
anyway to be sure.
DRAW LENGTH FITTING AND ADJUSTMENT: This
is the most important part of bow tuning. Getting the draw
length of the bow set to match the shooter is vital to helping
Using a D-loop on the bowstring is essential to trapping the nock
above and below, preventing it from sliding down the bowstring
during the power stroke. The Lark’s Head knot is shown here; the
top knot should face the opposite direction from the bottom knot
so the loop does not “chase” around the bowstring from shot to
shot.
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This photo demonstrates the most important aspect of shooting
form: proper full draw position for shooting archery. With today’s
bows, adjusting bow draw length until an archer’s drawing forearm and elbow are in line with the arrow is easy to do and see.
This position of the drawing arm allows the archer to transfer the
bow-holding load out of the arm and into the back muscles, thus
relaxing the forearm for a repeatable release and a natural followthrough. Any other position requires the use of more arm muscle,
resulting in a far less repeatable shot.
that shooter become a consistent and accurate archer. A bow
that is not matched correctly and is too short or too long will
contribute to poor shooting form, poor arrow groupings and
non-vital hits on game animals.
Just yesterday, I reviewed about 30 video clips on
YouTube and other websites. They attempted to give tips
about proper bow fitting to hunters and archers in general.
None of them and I do mean none got it correct.
All I heard from them was that the shooter should get
into a comfortable anchor and hold the bow arm toward the
target. But the term “comfortable anchor” has no definition;
I don’t know what that means. We need to define our terms
and since you use your body, muscles and skeleton, to shoot
a bow, you must define your proper full draw position or
“holding position” in terms that make your body biomechanically efficient.
You can use lots of muscle to shoot archery but if you do,
you’ll fatigue quickly and perform inconsistently. Instead,
you need to put your skeleton in position to carry the bowshooting load and relax most of your muscles in order to
reach your highest level of consistency and accuracy.
Here’s the position as I’ve presented it many times in
previous issues of ArrowTrade. It is also the position that is
prescribed by national training systems.
BUILDING FULL DRAW POSITION:
a) You should have your bow arm extended so that
the arm bones are in line and resist the force of the bow;
they should act like a stick between your body and the bow
handle.
b) Your drawing (holding) forearm and elbow should be
positioned so that they are in line with the arrow (from a top
view), enabling you to transfer the bow-holding force into
your back muscles on your holding side. This allows you to
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relax your holding forearm and upper arm in order to affect
your most consistent release and follow-through.
c) Your shoulder line should be rotated so that it is near
parallel to the arrow and your skeleton can carry the load of
the held bow.
Drawing short or long relative to this line will render
an archer’s back muscles less effective, less consistent and
therefore less accurate at the moment of truth. I realize that
homespun methods are well intended but if you ignore the
science of biomechanics, you and your customers will pay
the price. Line up the drawing forearm with the arrow and
you’ll shoot your best.
DRAW LENGTH ADJUSTMENTS: Fitting archers is easy
if you have the previously mentioned holding position
model to use as a reference. I know there are some mathematical formulas that many use but they do not give correct values for the vast majority of archers. None of those
rules get within an inch of the correct value for me. As I was
a mathematics educator for 35 years, I can tell that someone
made up those formulas by choosing numbers at random;
this is not how to make a rule or formula that gives accurate
values. Besides, what standard for holding position were
those people using anyway? These rules are why 70 percent
of the students I work with or the archers I see at tournaments are in a “short” holding position and using too much
arm muscle to shoot archery and get their highest scores.
If your customer is drawing short of the prescribed
arrow line, then increase the draw length of the bow by
changing the draw modules, increasing the D-loop length,
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adding twists to the cables or removing twists from the
string. The last resort, of course, is to install a longer draw
cam system, which is not usually cheap.
If your customer is drawing too long, with the holding
arm beyond the arrow line, then shorten the draw length
by changing the modules, shortening the D-loop if possible,
removing twists on the cables, adding twists on the string or,
lastly, installing a smaller cam system.
Just last week, at the Illinois Deer Expo, I encountered a
man who was 6 feet, 8 inches tall. I knew his question before
he asked it: “How can I get a bow to fit me?” To that point in
time, he had never been able to extend his bow arm or align
his holding arm properly. I suggested that I would lengthen
my PSE Freak SP, a 38 inch axle-to-axle bow, to its longest
setting and see how it fit; we could make a plan from there.
My display bow was equipped with a Safe Draw air
piston so he could draw and shoot it right at my booth. To
my surprise, it fit perfectly; he could align properly. Upon
To determine if arrow fletching is making contact with some part
of the arrow rest, you need to spray the fletching with some white
foot powder. Shoot the arrow and then inspect the powder for
contact markings. Rotating the arrow nock and/or changing the
nocking point location will eliminate the contact in most cases.
Using a paper covered frame is a great test for arrow flight. Hang
the frame about 5 feet in front of your target butt, stand back at
5 to 8 yards and shoot an arrow through the paper. The hole will
show you how the arrow begins its flight path out of your bow. If
the nock shows a high tear, then try lowering the nocking point on
the bowstring.
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measuring it, we found that it was actually giving him 33.75
inches of draw length. Make this example a guide: a person
who is 6 feet, 8 inches tall needs about 33.75 inches of draw
length, depending on the individual.
POWDER TEST: Spray white powder foot spray on the
fletched end of an arrow, shoot it and check for contact
marks on the fletching. If it is contacting the arrow rest, then
make adjustments to the center shot, nocking point and/
or nock rotation in order to eliminate the contact. Be aware
that some drop-away rests bounce upward, into the path of
the arrow and may need adjustment to allow the rest to fall
completely out of the fletching path.
PAPER TESTING: I use this test, an arrow shot through
a piece of newspaper, to verify that I have an arrow size that
is reasonably close to the correct size for a bow setup. A bad
tear in the paper from 5 yards of distance will tell you rather
quickly that something is not good. If you’ve followed the
factory guidelines on rest setup, then you can be reasonably
certain that the arrow size is the source of the problem. Try
several other sizes until you get a reasonable paper tear.
What kind of tear is reasonable is the question. When
shooting from 5 yards, I expect to get a hole that is less than
1/2 of an inch long. I prefer the hole, if it’s not near perfect,
to be slightly high left (for right-handers) or just plain high.
A high tear indicates the need for a lower nocking point
while a low tear shows a need for a higher nocking point.
Left tears by a right-hander indicate the need for a weaker
spined shaft and/or more draw weight and/or the center
shot moved right. The opposite is true for right tears.
CABLE GUARD CLEARANCE AND FLEX: The first
cable guard that I saw on a compound
bow was my 1979 Jennings Split T two-wheeler. It looked like
an aluminum flyswatter. It flexed toward the bowstring as
the bow was drawn and the cables exerted torque. I remember that bow shot well right out of the box and within a few
weeks, I shot a 558 practice field round at my home club.
A look at the cable guards on bows today makes me
think we’re right back where Tom Jennings started us: flexible cable guards. Many now bend a little under the cable
load while others can be adjusted to change the distance
between the cables and bowstring and therefore the torque
load on the bow handle and how the arrow leaves the bow.
In other words, if the arrow is leaving with a nock-right attitude, then a little adjustment of the distance between the
cable guard and bowstring may straighten the arrow flight.
SHORT RANGE SHOOTING: Bear Archery recommends that actual shooting start at 15 yards. I also like that
distance. Twenty yards will do as well and I’ve started at 10;
it all depends on how confident you are in your initial sight
installation. Adjust the sight until the arrows are grouping in
a 3 inch spot at close range.
In order to fine-tune the arrow rest center shot location,
use the plumb (vertical) line test. From a 3 inch aiming spot,
hang a plumb line or strip of masking tape. Shoot one arrow
at 15 yards at the aiming spot.
Next, step back 10 yards, aim at the same spot with
the same pin and shoot one arrow. This arrow will impact
slightly below the first arrow. Move 10 yards further from the
target and shoot a third arrow, resulting in three arrows in a
nearly vertical line. If the arrows are near the plumb line, all
is well with the center shot location of your arrow rest. If the
The flexibility and location of the cable
guard affect arrow flight. As the guard
gets closer to the bowstring, it applies less
torque to the handle riser in the horizontal
plane, thus affecting the arrow and the
aiming characteristics of the bow system.
Use this as part of your tuning process.
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lower arrows impact further to one side of the plumb line,
then adjust the horizontal position arrow rest; arrows to the
left of the line indicate the need for a rest adjustment to the
right and vice versa.
Repeat the plumb line test until you get consistent
results. Target shooters often use this test at much longer
distances; skill level determines the maximum distance a
given archer should use.
LONG RANGE TESTING: Bow tuning does not end until
you have the best arrow groups your ability allows you to
produce. That means that you must shoot from a long range,
assess the groups and make any needed adjustments to
make improvements. I’ve spent lots of hours shooting at 50
and 60 yards, with many excursions to 80 yards that I needed
for field archery. Here are some of the techniques I employ:
WHEEL TIMING: I test small changes in wheel timing to
gauge aiming steadiness and group tightness. Your students
can do this by adding or removing single twist increments to
the yoked power cable on an asymmetric cam bow. Record
the results. Again, the student is looking for steadiness and
repeat feel for the shot.
TILLER: Small changes to tiller settings may have an
effect on how the bow sits on the aiming spot. Your student
should add 1/4 of a turn to one limb bolt and remove 1/4
of a turn from the opposite bolt, then shoot test. Record the
results while looking for the best and steadiest hold on the
spot center.
VERTICAL GROUPINGS: Some bow setups produce
high and low misses at the target. For groups that appear
as a vertical line of arrows through the target spot, check
the following:
High nocking point: Lower the nocking point on the
string and retest.
Weak launcher: Stiffen the launcher in some manner by
making it longer, narrower or thinner.
Heavy point: Test a lighter point.
Low hand pressure: Reset the bow hand to apply even
pressure instead of “heel” pressure, then retest.
Loose nock: Check for proper nock fit. Also check to be
sure the nock is trapped above and below, as nock-slide on the
string causes high arrows.
HORIZONTAL GROUPINGS: Other bow setups produce horizontal groupings across the middle of the target
spot. Check the following:
Low nocking point: Raise the nocking point on the string
and retest.
Stiff launcher: Weaken the launcher in some manner
and retest.
Light point: Test a heavier point in your arrows.
Hand torque: The hand is tensed in some way and applying torque to the bow grip. Reposition and relax the bow hand
and retest.
Tight nock: Modify the nock fit and check the D-loop for
nock-pinch.
WILD ARROWS: Occasionally, a bow setup will produce wild arrows that don’t impact with the others in a
group. Check the following:
Bad arrow: The arrow may be defective in some way: a
crack, a bad nock, it is not straight, a bent point or an invisible sidewall flaw. Set it aside and test it again later.
Fletch contact: Spray test the fletched end again to check
for arrow rest contact and adjust accordingly.
Mismatched spine: The arrow spine being used is not
matched to the bow setup. Try other shaft sizes and spines.
Wheel timing: The twin or asymmetric cam system is
slightly out of adjustment. Use cable twisting techniques to
reset the timing for best groups.
CONCLUSION
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This tuning guide is a good way to get started but by no
means is it complete. I’m sure you have other techniques
that you use to get your customers’ bows in good shooting
order; add them to this list. The bow designs of the last five
years make it relatively easy to get good arrow flight and tight
groups so dealers like you don’t have to spend lots of valuable selling time helping someone tune a bow.
Don’t forget to recommend this article (it’s online now
at arrowtrademag.com) to all of your customers so they
can learn on their own and help you teach others to help
themselves.
Shoot straight, keep well.
Larry
Editor’s Note: Larry’s latest book, “Larry Wise on
Coaching Archery,” is now available. This book is a compilation of over 25 ArrowTrade Magazine articles that Larry has
written and it can be purchased at www.larrywise.com or
through Archery Focus, an online magazine.
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